Heavy fuel oil

Heavy fuel oil is a fuel oil of a tar-like consistency. Also known as bunker fuel, or residual fuel oil, HFO is the residual mixture leftover from the distillation and cracking of crude oil in oil refineries. Generally, it has a boiling temperature between 350 and 500 °C and a significantly increased viscosity compared to diesel. As it is created through the extraction of more valuable components of its petroleum precursor, HFO contains various undesirable compounds and elements, which includes aromatics, sulfur, nitrogen, vanadium, and others. These non-hydrocarbon contaminants significantly increase toxic gas and particulate emissions upon combustion, such as sulfur dioxide, carbon monoxide, and nitrogen oxides.
As the fuel is cheap, it is predominantly utilized for marine vessel propulsion in marine diesel engines due to its relatively low cost compared to cleaner fuel sources such as diesel fuel or distillates. The emission-heavy nature of the fuel also contributes to this method of usage; marine vessels, such as oil tankers and cruise ships, are generally distant from population centers, sailing in open seas and oceans for the majority of the time, minimizing the exposure of humans to harmful aerosols and gaseous emissions. Ships utilizing heavy fuel oil may switch to cleaner alternatives such as diesel when approaching land. The use and carrying of HFO in seafaring vessels presents several environmental concerns, such as accidental oil spills due to adverse weather or routine handling, which are common due to their universal and dominant usage in marine transportation.
After the International Maritime Organization implemented a global sulfur emissions cap in 2020, a growing number of ships have been equipped with scrubbers, which allow ships to continue high-sulfur heavy fuel oil use while meeting air quality regulations, shifting the environmental burden from air to water.
The use of HFOs is banned as a fuel source for ships travelling in the Antarctic as part of the International Maritime Organization's International Code for Ships Operating in Polar Waters. For similar reasons, an HFO ban in Arctic waters is currently being considered.

Heavy fuel oil characteristics

HFO consists of the remnants or residual of petroleum sources once the hydrocarbons of higher quality are extracted via processes such as thermal and catalytic cracking. Thus, HFO is also commonly referred to as residual fuel oil. The chemical composition of HFO is highly variable due to the fact that HFO is often mixed or blended with cleaner fuels; blending streams can include carbon numbers from C₂₀ to greater than C₅₀. HFOs are blended to achieve certain viscosity and flow characteristics for a given use. As a result of the wide compositional spectrum, HFO is defined by processing, physical and final use characteristics. Being the final remnant of the cracking process, HFO also contains mixtures of the following compounds to various degrees: "paraffins, cycloparaffins, aromatics, olefins, and asphaltenes as well as molecules containing sulfur, oxygen, nitrogen and/or organometals". HFO is characterized by a maximum density of 1010 kg/m³ at 15 °C, and a maximum viscosity of 700 mm²/s at 50 °C according to ISO 8217.

Combustion and atmospheric reactions

Given HFO's elevated sulfur contamination, the combustion reaction results in the formation of sulfur dioxide SO₂.

Heavy fuel oil use and shipping

Since the middle of the 20th century, HFO has been used primarily by the shipping industry due to its low cost compared with all other fuel oils, being up to 30% less expensive, as well as the historically lax regulatory requirements for emissions of nitrogen oxides and sulfur dioxide by the IMO. For these two reasons, HFO is the single most widely used engine fuel oil on-board ships. Data available until 2007 for global consumption of HFO at the international marine sector reports total fuel oil usages of 200 million tonnes, with HFO consumption accounting for 174 million tonnes. Data available until 2011 for fuel oil sales to the international marine shipping sector reports 207.5 million tonnes total fuel oil sales with HFO accounting for 177.9 million tonnes.
Marine vessels with combustion engines can use a variety of different fuels for the purpose of propulsion, which are divided into two broad categories: residual oils or distillates. In contrast to HFOs, distillates are the petroleum products created through refining crude oil and include diesel, kerosene, naphtha and gas. Residual oils are often combined to various degrees with distillates to achieve desired properties for operational and/or environmental performance. Table 1 lists commonly used categories of marine fuel oil and mixtures; all mixtures including the low sulfur marine fuel oil are still considered HFO.

Category of marine HFO	Marine HFO composition
Bunker C/Fuel oil No.6	Residual oil
Intermediate Fuel Oil 380	Distillate combined with 98% residual oil
Intermediate Fuel Oil 180	Distillate combined with 88% residual oil
Low Sulfur Marine Fuel Oils	Distillate/residual oil blend

Arctic environmental concerns

The use and carriage of HFO in the Arctic is a commonplace marine industry practice. In 2015, over 200 ships entered Arctic waters carrying a total of 1.1 million tonnes of fuel with 57% of fuel consumed during Arctic voyages being HFO. In the same year, trends in carriage of HFO were reported to be 830,000 tonnes, representing a significant growth from the reported 400,000 tonnes in 2012. A report in 2017 by Norwegian Type Approval body DNV GL calculated the total fuel use of HFO by mass in the Arctic to be over 75% with larger vessels being the main consumers. In light of increased area traffic and given that the Arctic is considered to be a sensitive ecological area with a higher response intensity to climate change, the environmental risks posed by HFO present concern for environmentalists and governments in the area. The two main environmental concerns for HFO in the Arctic are the risk of spill or accidental discharge and the emission of black carbon as a result of HFO consumption.

Environmental impacts of heavy fuel oil spills

Due to its very high viscosity and elevated density, HFO released into the environment is a greater threat to flora and fauna compared to distillate or other residual fuels. In 2009, the Arctic Council identified the spill of oil in the Arctic as the greatest threat to the local marine environment. Being the remnant of the distillation and cracking processes, HFO is characterized by an elevated overall toxicity compared to all other fuels. Its viscosity prevents breakdown into the environment, a property exacerbated by the cold temperatures in the Arctic resulting in the formation of tar-lumps, and an increase in volume through emulsification. Its density, tendency to persist and emulsify can result in HFO polluting both the water column and seabed.

Category of marine HFO	Immediate spill impact	Environmental impact	Cleanup characteristics
Bunker C/Fuel oil No.6	May emulsify, form into tar balls, remain buoyant or sink to the seabed.	Tar-like consistency of HFO sticks to feathers and fur, results in short and long term impacts on marine flora and fauna	Water recovery of spill is limited, cleanup consists mainly of shoreline and oiled substrate remediation.
Intermediate Fuel Oil 380	Emulsifies up to 3x the original spill volume, may sink to seabed or remain buoyant.	Tar-like consistency of HFO sticks to feathers and fur, results in short and long term impacts on marine flora and fauna	Skimmers are used to recover on-water spill until the oil emulsifies making its removal more difficult. Once coated to the surface, the oil is difficult to remove from substrate and sediment.
Intermediate Fuel Oil 180	Emulsifies up to 3x the original spill volume, may sink to seabed or remain buoyant.	Tar-like consistency of HFO sticks to feathers and fur, results in short and long term impacts on marine flora and fauna	Skimmers are used to recover on-water spill until the oil emulsifies making its removal more difficult. Once coated to the surface, the oil is difficult to remove from substrate and sediment.
Low Sulfur Marine Fuel Oils	No ground data to determine immediate spill impact. Laboratory tests suggest behavior similar to other HFO mixtures namely environmental persistence and emulsification.	Limited information. Likely to have similar impacts as IFO with increased initial toxicity due to the higher distillate component causing immediate dispersal and evaporation.	Limited information. Likely to have similar impacts to other HFO mixtures.

History of major HFO spills from 2000 onward

For a more complete list, see the List of oil spills.
The following HFO specific spills have occurred since the year 2000. The information is organized according to year and ship name and includes amount released and the spill location:

Date	Link/Reference	Vessel	Flag/State	Location	Tonnes	Notes
2000		Janra	Germany	Sea of Åland	40
2001		Baltic Carrier	Marshall Islands	Baltic Sea	2350	-
2002	Prestige oil spill	MV Prestige	Athens, Greece	Spain, Atlantic Ocean	60000
2003		Fu Shan Hai	China	Baltic Sea	1680
2004		Selendang Ayu	Malaysia	Unalaska Island - near Arctic	1300
2007	Cosco Busan oil spill	Cosco Busan	Germany	San Francisco Bay	191	IFO 380
2009	Full City oil spill	Full City	Panama	Langesund, Norway	200
2011-02-17		Godafoss	Iceland	Hvaler Islands, Norway	125
2011		Golden Trader	Panama	Skagerrak, Denmark	205	IFO 180